This invention relates generally to multi-beam electron guns as used in color cathode ray tubes (CRTs) and is particularly directed to a multi-layer common lens arrangement in one or more charged grids in the main focus lens of a CRT electron gun.
A typical color CRT employs a multi-beam electron gun which directs three inline electron beams on the inner surface of the CRT""s glass display screen. A magnetic deflection yoke disposed outside of the CRT""s glass envelope sweeps the three electron beams in unison across the display screen in a raster-like manner. The three electron beams are aligned generally horizontally, or in the direction of each sweep across the CRT""s display screen. The energetic electrons incident upon a phosphor coating disposed on the display screen""s inner surface produce a video image.
Electron guns are characterized as having X-, Y-, and Z-axes respectively aligned along the width, height and length of the electron gun structure. These axes are shown in FIG. 1 which is a longitudinal sectional view of a prior art bipotential inline electron gun 10 incorporating a common lens arrangement in its main focus lens. The Y-axis aligned with the height of the bipotential inline electron gun 10 is perpendicular to the plane of the drawing sheet. In general, the larger the electron gun is along its X- and Y-axes, or the larger its diameter, the better the resolution of the video image presented on the CRT""s display screen. Over the past several years, the design of high resolution color CRT electron guns has evolved from the individual beam main lens design to the common lens design for the purpose of increasing the effective size of the electron gun. In the individual beam type of main lens design, each of the three electron beams (red, blue, green) is directed through an individually defined lens space without sharing the space with the other beams. In the common lens design, each of the three electron beams is directed through its own individual beam path as well as through a shared focusing region defined by a common beam passing aperture.
Referring to FIG. 1, there is shown a longitudinal sectional view of a prior art bipotential inline electron gun 10 incorporating a common lens arrangement in its main focus lens. Electron gun 10 includes an electron beam source typically comprised of three cathodes: KR (red), KG (green) and KB (blue). Each cathode emits electrons which are focused to a crossover along the axis of the beam by the effect of an electrode commonly referred to as the G2 screen grid. An electrode known as the G1 control grid is disposed between the cathodes and the G2 screen grid and is operated at a negative potential relative to the cathodes and serves to control the intensity of the electron beams in response to the application of a video signal to the cathodes. Each of the G1 control and G2 screen grids includes three respective aligned apertures 12a, 12b, 12c and 14a, 14b, 14c, with corresponding apertures in each electrode in common alignment for passing a respective one of the red, green or blue color generating electron beams. The G2 screen grid is connected to and charged by a VG voltage source 33.
Electron gun 10 further includes a G3 electrode and a G4 electrode disposed about the three electron beams and along the path of the energetic electrons as they travel toward a display screen 40 disposed on a forward portion of the CRT""s glass envelope (which is not shown in the figure for simplicity). The G3 grid is connected to and charged by a VF focus voltage source 34, while the G4 grid is coupled to and charged by a VA accelerating, or anode, voltage source 35. The lower end of the G3 grid in facing relation to the G2 screen grid forms, in combination with the G1 control grid and the G2 screen grid, a beam forming region for forming the three groups of energetic electrons emitted by the KR, KG and KB cathodes into three spaced electron beams. The lower end of the G3 grid includes three inline, spaced apertures 16a, 16b and 16c through each of which is directed a respective electron beam.
While the G1 control and G2 screen grids are generally flat, the G3 grid and a G4 grid are cup-like in shape. Disposed within the G3 grid is a second trio of beam passing apertures 20a, 20b and 20c, through each of which is directed a respective one of the electron beams. The G3 and G4 grids form the electron gun""s main focus lens. Disposed on the upper portion of the G3 grid in facing relation to the G4 grid is an elongated common beam passing aperture 18 through which all three electron beams are directed. Beam passing aperture 18 extends substantially the entire width and height of the G3 grid and typically has a chain link shape. This chain link shape includes three spaced curvilinear enlarged portions through each of which is directed a respective one of the electron beams. This chain link shaped common beam passing aperture is shown in figures discussed in the following paragraphs and is described in detail below. The common beam passing aperture may take on other common forms, e. g., race track, dog bone or elliptical, although these other shapes are not shown in the figures for simplicity.
The G4 grid also includes an elongated common beam passing aperture 22 in facing relation to the beam passing aperture 18 of the G3 grid. Disposed within the G4 grid in spaced relation are three inline beam passing apertures 24a, 24b and 24c through each of which is directed a respective one of the electron beams. Disposed on the upper end portion of the G4 grid is a conductive support, or convergence, cup 26 which includes plural bulb spacers 28 disposed about its circumference in a spaced manner. The support cup 26 and bulb spacer 28 combination is conventional and serves to securely maintain electron gun 10 in position in the neck portion of a CRT""s glass envelope. Each of the aforementioned grids is coupled to and supported by glass beads (also not shown for simplicity) disposed in the glass envelope""s neck portion.
After being subjected to the electrostatic fields produced by the accelerating and focusing voltages applied by the aforementioned grids, the focused electron beams are then directed through a magnetic deflection yoke 30 for deflecting the electron beams in a raster-like manner across a phosphor coating, or layer, 40 on the inner surface of the CRT""s display screen, or glass faceplate, 42. Disposed adjacent the inner surface of the CRT""s display screen 42 is a shadow mask 36 having a larger number of apertures 36a therein and serving as a color selection electrode.
By directing all three electron beams through a common beam passing aperture, the effective width and height, i.e., diameter, of the electron gun is increased to provide improved video image resolution. Because the electron gun is disposed within the narrow neck portion of the CRT""s glass envelope, the common lens design overcomes prior limits on the size, i.e., height and width, of the individual lens-type electron gun.
The length of the electron gun along its Z-axis may also be increased. However, increasing the length of the electron gun along its Z-axis creates a large asymmetric astigmatism which reduces video image resolution. Electron beam astigmatism is defined in terms of the difference between the horizontal focus voltage and the vertical focus voltage, or:
astigmatism=VFHxe2x88x92VFV
where
VFH=horizontal focus voltage, and
VVF=vertical focus voltage.
The present invention addresses the aforementioned limitations of the prior art by increasing the effective electrostatic focusing field applied to the electron beams by increasing the effective diameter of the electron gun and compensating for this increase in size by increasing the gun""s length. By electrostatically compensating for the electron gun""s increased effective diameter, electron beam astigmatism is also compensated for and video image resolution is improved.
Accordingly, it is an object of the present invention to provide improved electron beam focusing in a multi-beam electron gun such as incorporated in a color CRT.
It is another object of the present invention to electrostatically increase the effective diameter of the main focus lens of an electron gun to compensate for increased electron gun length without increasing electron beam astigmatism for improved electron beam focusing on the display screen of a CRT.
Yet another object of the present invention is to provide a layered common lens arrangement in a multi-beam electron gun including one or more charged grids each having plural common apertures through which the electron beams are directed for improved focusing of the electron beams on a display screen upon which a video image is presented.
A further object of the present invention is to compensate for electron beam astigmatism in a video image produced by plural electron beams directed by an electron gun on a display screen such as in a color CRT, where the astigmatism arises from increasing the length of the electron gun without increasing the electron gun""s diameter.
A still further object of the present invention is to improve resolution of a video image produced by plural electron beams directed by an electron gun onto a display screen by increasing the electron gun""s length without increasing its diameter or the focus voltage.
The present invention contemplates a charged electrode in an electron gun forming an electrostatic focusing field for focusing plural electron beams on a display screen of a color cathode ray tube (CRT) in forming a video image on the screen, wherein the plural electron beams are directed along respective parallel axes, the electrode comprising a hollow housing including a first wall for defining three inline apertures and a thin side wall forming lateral portions of the housing, wherein each of the inline apertures is aligned with a respective one of the axes for passing a respective one of the electron beams; plural second walls disposed in the hollow housing and extending inwardly toward the electron beam axes from the side wall, wherein the plural second walls are disposed in a spaced manner along the electron beam axes; and an elongated common aperture in each of the second walls, wherein the common apertures are aligned in a spaced manner along the electron beam axes and the electron beams are directed through the aligned common apertures, and wherein the plural walls increase the effective radius of the electrostatic focusing field of the electrode and the length of the electrostatic focusing field along the axes for improved electron beam focusing on the display screen.